Fundamentals of Acids & Bases (A Level only) (AQA A Level Chemistry)

This question is about Brønsted-Lowry acids and bases.

When an acid and a base react they produce a conjugate base and a conjugate acid.

acid + base  ⇋  conjugate base + conjugate acid

Write an equation to show how hydrochloric acid behaves as a strong acid when it reacts with water, and state the role of water in this reaction.

The pH of an acid or base can be calculated using the concentration of hydrogen ions.

Acids can be classed as monoprotic, diprotic and triprotic.

At 298K, water molecules dissociate into equal quantities of ions, and the pH is 7.

The ionic product of water, K_w, can be used to find the pH of a strong base. Changing the temperature will affect the value for K_w.

At 40 C the K_w of pure water is 2.916 x 10^-14 mol² dm^-3.

Calculate the pH of pure water. Give your answer to 3 significant figures.

Strong bases fully ionise in water, as shown by the equation of dissociation of sodium hydroxide:

NaOH (aq) →  Na⁺ (aq) + OH^- (aq)

At 298K, K_w is 1 x 10^-14 mol² dm^-6.

Calculate the pH of a 0.05 mol dm^-3 solution of NaOH at 298 K. Give your answer to 3 significant figures.      

Weak acids don’t fully ionise in solution. The equilibrium constant, K_a is used to determine the hydrogen ion concentration.

Write an expression for the acid dissociation constant, K_a for the acid HA.

A sample of 0.01 mol dm^-3 butanoic acid has a K_a value of 1.51 x 10^-5 mol dm^-3.

The pH of a 0.15 mol dm^-3 solution of HCN is 5.08 at 298K.

Determine the value of K_a for HCN at 298 K.

Give your answer to two decimal place and state its units.

The value of K_a can be used to determine the strength of an acid. The lower the value for K_a the weaker the acid is. Hydrofluoric acid has a K_a value of 5.5 x 10^-4. In comparison, the K_a value for ethanoic acid is 1.7 x 10^-5.

Considering the expression for K_a,  explain why the value of K_a is larger in hydrofluoric acid than in ethanoic acid.

pK_a values can be used to determine the strength of an acid.

Give the expression for pK_a.

Propanoic acid has a pK_a of 4.87 at 298K.

Calculate the pH of 0.025 mol dm^-3 propanoic acid. Give your answer to three significant figures.

Formic acid is a weak monoprotic acid. At 298K the pH of 0.025 mol dm^-3 formic acid is 5.4.

Calculate the pK_a of formic acid at 298 K. Give your answer to three significant figures.

The pH curve shown in Figure 1 was obtained when a 0.150 mol dm^–3 solution of sodium hydroxide was added to 25.0 cm³ of an aqueous solution of ethanoic acid.

The half equivalence point is where half of the volume of sodium hydroxide required for neutralisation has been added to the ethanoic acid.

Label the graph with an X to show the position of the half equivalence point.

Figure 1

In its purest form at 25 C, water has a pH of 7.

At 50 C, its pH is 6.63. A student thought that the water would be slightly acidic if it had a pH of 6.63 at this temperature.

The ionic product of water, K_w is an equilibrium constant.

However, the expression for K_w does not include the concentration of water.

Explain why.

Using suitable equations, explain why nitric acid, HNO₃, is classed as a strong acid but ethanoic acid, CH₃COOH is classed as a weak acid.

A student measured 50.0 cm^–3 of 0.025 mol dm⁻³ hydrochloric acid into a beaker, A. The student then added 100 cm^–3 of distilled water into the beaker.

Calculate the pH of the solution formed after 100 cm^–3 of distilled water are added to beaker A. Give your answer to 3 significant figures.

3.5 g of potassium hydroxide, KOH (aq), is added to 250 cm³ of water. State the pH of the solution at 298 K. Give your answer to 1 decimal place.

(K_w = 1.00 x 10^-14 mol² dm^-6 at 298 K)

20.00 cm³ of 0.01 mol dm^-3 hydrochloric acid, HCl (aq), is added to 50.00 cm³ of the solution of potassium hydroxide given in part (a). State the new pH of this solution.

Give your answer to 2 decimal places.

(If you have been unable to calculate an answer for (a), you may assume a value of 0.4 mol dm^-3 as the concentration of KOH. This is not the correct answer)

At 45 C the value for K_w changes to 4.02 x 10^-14 mol dm^-3.

7.00 cm³ of 0.25 mol dm^-3 potassium hydroxide solution is added to 993 cm³ of pure water at 316 K. Using the value for K_w in part (c), calculate the pH of the solution to 2 decimal places.

Sulfuric acid, H₂SO₄, is a diprotic acid. Assuming the sulfuric acid fully ionises the pH of a solution of 0.0021 moles in 42 cm³ of sulfuric acid to 2 decimal places.

A student stated that ‘as temperature increases the pH value of pure water decreases.

State whether the student is correct. Justify your answer. 

Explain why an ammonium ion can not behave as a Brønsted-Lowry base.

State and explain the acid-base character of aqueous ammonia at 298 K.

32.00 cm³ of 0.30 mol dm^-3 hydrochloric acid is added to 30.00 cm³ of 0.50 mol dm^-3 sodium hydroxide. Calculate the pH of the resulting solution at 298 K. Give your answer to 2 decimal places.

(K_w = 1.00 x 10^-14 mol² dm^-6 at 298 K)

20.00 cm³ of 0.40 mol dm^-3 barium hydroxide solution, Ba(OH)₂ (aq), is added to the 35.00 cm³ of 0.30 mol dm^-3 of hydrochloric acid. What is the pH of the new solution at 298 K?

Give your answer to 2 decimal places. 

(K_w = 1.00 x 10^-14 mol² dm^-6 at 298 K)

Distilled water was added to 30.0 cm³ of 0.075 mol dm^-3 nitric acid, HNO₃ (aq), until the pH reached 1.4. What is the total volume of the solution in dm³?

The distilled water was heated to 50 C and the pH decreased to 6.63. Calculate the value K_w at this temperature to 2 decimal places, including the units.

At 15 C 27.75 cm³ of lithium hydroxide, LiOH (aq), contains 1.11 x 10^-3 moles of lithium hydroxide and has a pH of 12.95. What is the value for K_w at 15 C.

Table 1 gives values for the ionic product of water at two different temperatures.

Table 1

Explain why the value of K_w is affected by temperature.

Acids can be classed as monoprotic, diprotic and triprotic. Sulfuric acid is a diprotic acid. 

Suggest why the first ionisation step of sulfuric acid goes to completion and the second ionisation step reaches equilibrium.

At 25 C the value for the dissociation constant of water is K_w is 1.00 x 10^-14 mol² dm^-6.

Propanoic acid can behave as a weak acid.